Trying to understand RSTP - Please can someone explain this?

Unanswered Question
Feb 6th, 2009

Hi Group

I am still confused about how RSTP is implemented. From what i understand the major difference is that STP

used Timers for Loop prevention whereas RSTP coordinates between neighbors via messages (proposal/aggreement) to turn on links

more quickly after topology changes and is "timer free".

However, I have not noticed any difference in the configuration from the legacy STP

configurations and the RSTP configuration on cisco devices. Or are there any differences??

I have read in documentation that RSTP natively includes features like UplinkFast, BackboneFast and PortFast. So are these features now obsolete

and not needed to be configured if you are running RSTP. (Although i have seen Portfast still configured along with RSTP on many switches)

Also can someone explain the below Points from Cisco Documentation

1) should STP be disabled on edge ports all together as suggested below?

"STP edge ports are bridge ports that do not need STP enabled, where loop protection is not needed out

of that port or an STP neighbor does not exist out of that port. For RSTP, it is important to disable STP

on edge ports, which are typically front-side Ethernet ports, using the command bridge

bridge-group-number spanning-disabled on the appropriate interface. If RSTP is not disabled on edge

ports, convergence times will be excessive for packets traversing those ports."

2) It seems RSTP relies on duplex setting to determine inter-switch links. What is the configuration to explicitly

configure RSTP link types? (I couldnt find this in the documentation)

"RSTP can only achieve rapid transition to the forwarding state on edge ports and on point-to-point links.

The link type is automatically derived from the duplex mode of a port. A port that operates in fullduplex

is assumed to be point-to-point, while a half-duplex port is considered as a shared port by

default. This automatic link type setting can be overridden by explicit configuration. In switched

networks today, most links operate in full-duplex mode and are treated as point-to-point links by RSTP.

This makes them candidates for rapid transition to the forwarding state."

Also i am a bit rough on my RSTP knowledge even after skimming a few Cisco documents. Can someone please explain this in simple way.

Thanks in advance

I have this problem too.
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siddiqirf Fri, 02/06/2009 - 05:23

I have read these documents, they dont explain it very well unfortunately.

OK.

RSTP enhances 802.1d with one goal in mind, to improve STP convergance. RSTP defined new variations on BPDU's between switches, new port states and new port riles, all with the capability to operate backwardly compatible with 802.1d switches. The key things of speeding convegance with 802.1w is:-

1) Waiting only for three missed Hellos on an RP before reacting (version 10 missed hellos via the maxage timer in 802.1d)

2) New processes that sllow transition from the disabled state (replaces the blocking state in 802.1d) to a learning state, bypassing the the 802.1d listening state.

3) Standardisation of features like Cisco PortFast, UplinkFast and BackboneFast

4) An Additional feature to allow a backup DP when a switch has multiple ports connected to the same shared LAN segment.

5) Using different link types to describe ports, like Point-to-point, Shared, and Edge.

6) Fewer port states, instead of the 802.1d 5 states, 802.1w has only 3 states.

HTH>

siddiqirf Fri, 02/06/2009 - 06:49

Thanks Can you explain how configuration on the switch will differ on the CLI ? Doest this mean you no longer need to configure Portfast, uplinkfast, etc

Also if you can answer the two questions i posted on my original message.

Many thanks

to configure it on a device:-

spanning-tree mode rapid-pvst

PortFast/UplinkFast & BackboneFast were cisco "Enhancements" to 802.1d STP. RSTP just incorperates them. If you want to configure portfast, the command is still "spanning-tree portfast"

OK

1) That is your choice - I have bitter experiance of users/IT admins just plugging hubs/switches in when ever they can. Also cabling the switch back to itself creating a cabled loop. So my advice to you is to leave STP enabled on all switch ports, BUT enable BPDUGuard - this is a life saver, if you have configured portfast.

2) duplex auto! or duplex full (overiding)

I really suggest that you read the 802.1d standard, once you understand normal spanning-tree - RSTP will come to you.

http://www.cisco.com/en/US/tech/tk389/tk621/tsd_technology_support_protocol_home.html')">http://www.cisco.com/en/US/tech/tk389/tk621/tsd_technology_support_protocol_home.html')">http://www.cisco.com/en/US/tech/tk389/tk621/tsd_technology_support_protocol_home.html')">http://www.cisco.com/en/US/tech/tk389/tk621/tsd_technology_support_protocol_home.html

http://en.wikipedia.org/wiki/Spanning_tree_protocol')">http://en.wikipedia.org/wiki/Spanning_tree_protocol')">http://en.wikipedia.org/wiki/Spanning_tree_protocol')">http://en.wikipedia.org/wiki/Spanning_tree_protocol

HTH>

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